Movable barrier systems typically include an operator that selectively moves a movable barrier (such as a segmented or one-piece garage door, swinging gate, sliding gate, rolling shutter, and so forth) between an opened and a closed position along guide tracks. Such barrier systems often include a counterbalance system, typically either a torsion spring counterbalance system or an extension spring counterbalance system.
A torsion spring counterbalance system includes a shaft (sometimes referred to as a jack shaft or torsion shaft), one or more torsion springs coiled around and connected to the shaft, and one or more drums connected to the shaft. Associated with each drum is a cable attached at one end to the drum (typically at a notch or slot in the drum), and at the opposite end to the lower region of the door.
As the door is opened, the torsion spring exerts a rotational force on the shaft. Rotation of the shaft causes the cables to be pulled up and wound about the drums. Through the cables, the spring pulls against the lower region of the door, in effect, reducing the weight of the door. This assists the user (when the operator system is in manual mode) or the motorized barrier operator (when in automatic mode) with opening of the door. Similarly, as the door is lowered, the cables unspool from the drums and extend down with the closing door.
During proper closing of the barrier, sufficient tension is placed on the cables to hold the cables against the external surfaces of the drums. However, various events can cause slack in a cable, resulting in the cable unspooling (or “jumping”) from the drum. For example, slack often occurs when the speed of the door is slower than that of the operator. This slowdown in the movement of the door often can be attributed to obstructions in the path of the door. Slack can also occur when a user attempts to manually open the door when the door is connected to the barrier operator. Abnormalities along the surface of the drum or guide track can also cause slack in the cable.
Slack in cables of movable barrier systems is particularly problematic. An unspooled (or “thrown”) cable can become entangled or fall from the drum, rendering the counterbalance system inoperative. Slack in a cable may also result in uncontrolled downward acceleration of the door when, for example, an obstacle previously obstructing downward movement of the door is removed.
Resetting of thrown cables is time consuming and expensive, resulting in downtime and often necessitating a service call from a trained technician. In addition to the cables, the counterbalance system usually must also be reset.
Thus, it is advantageous to detect slack in the cable during operation of the movable barrier system, particularly before the cable becomes entangled or falls from the drum. It is further advantageous to stop the barrier operator from driving the barrier in the downward direction upon detection of slack in the cable.
Previous devices used to detect slack in a cable include mechanical components that must maintain a constant contact with the cable in order to detect slack in the cable. In this way, as the cables are wound up and paid out during normal operation of the barrier, they continuously rub against the mechanical components of the detection devices. Other devices are spaced away from the cable but detect slack in cables only upon contact of the cables against the devices. In both of these approaches, the cables necessarily contact the detection devices. Because cables are typically abrasive (having been typically formed of multi-strand steel), this contact damages the detection devices over time.